1. Field of the Invention
The present invention relates generally to optical communications, and more specifically to optical fibers having low macrobending loss suitable for splicing and for the fabrication of optical fiber couplers.
2. Technical Background
A high performance optical telecommunication system carries high data rates over long distances with no electronic regeneration. For example, rates of 10 Gb/s or more over unregenerated distances of three to five hundred kilometers have been achieved. A high performance system may employ high power signal lasers, optical amplifiers, dispersion compensation devices, optical switching devices, and may use wavelength division multiplexing. Optical telecommunications systems are progressing toward higher speeds and longer span lengths, making the requirements for system components more and more arduous.
One such system component is an optical fiber coupler. Optical fiber couplers provide for coupling of optical signals between optical fibers, and are ubiquitous in the devices used in optical telecommunications systems. Optical fiber couplers may be made, for example, by heating and stretching a pair of coextending optical fibers to fuse and taper them. An optical signal traveling in one of the optical fibers is evanescently coupled into the other optical fiber in the fused region. Optical fiber couplers are used in a variety of devices to split and combine optical signals. For example, optical coupler may be used to divide optical power between two paths with a desired ratio (e.g. 1:1, 9:1). An optical fiber coupler may also be used as a WDM to combine optical pump radiation with an optical signal in an erbium-doped fiber amplifier.
As the requirements for the optical performance of optical fiber couplers become ever more stringent, the need to eliminate sources of optical loss becomes critical. One such source is loss due to macrobending in the unfused regions of the optical fibers. Optical fiber couplers are generally made to have relatively long (e.g. 2-5 m) lengths of optical fiber leading from the coupling region. When an optical fiber coupler is assembled in a device, these optical fiber leads are often bent with a small radius or coiled around a spool. This type of bending is known as macrobending, to distinguish it from microbending due to microscopic distortions of an otherwise straight fiber. Conventional optical fibers tend to have relatively high macrobending losses, giving the assembled device an unacceptably high loss.
While there exist optical fibers having low macrobending loss, many of these fibers suffer from relatively high variability in manufacture. In these optical fibers, the profile may be such that the optical properties of the fiber vary widely with variations in the mass of its overclad layer. Optical fibers that are relatively insensitive to minor variations in overclad thickness are highly desirable.
A WDM coupler is an optical fiber coupler device that combines or separates light having two different wavelengths (e.g. 980 nm and 1550 nm). For use in the fabrication of devices such as WDM couplers, optical fibers having low cutoff wavelengths are highly desirable. In general, macrobending loss at a wavelength of 1550 nm increases with decreasing cutoff wavelength, making it difficult to provide a fiber having both low macrobending loss and low cutoff wavelength.
Conventional optical fibers do not provide for the manufacture of optical fiber couplers with the desired performance. There remains a need for an optical fiber that exhibits low macrobending loss while also having low cutoff wavelength, low splice loss, low attenuation, and the ability to be fabricated into a low-loss optical fiber coupler. From the cost and process point of view, ease of manufacture and insensitivity of optical fiber properties to process variations are also highly desirable properties.